1. Field of the Invention
The present invention relates to a dielectric ceramic and a laminated ceramic capacitor, and more particularly, relates to a dielectric ceramic which is suitable for use in a thin-layer high-capacitance type laminated ceramic capacitor, and a laminated ceramic capacitor configured with the use of the dielectric ceramic.
2. Description of the Related Art
As one of effective means for fulfilling the demands of reduction in size and increase in capacitance for laminated ceramic capacitors, there is the reduction of an attempt to reduce the thickness of dielectric ceramic layers provided in the laminated ceramic capacitors.
Therefore, the reduction down to, for example, less than 1 μm has been recently required as the thickness of the dielectric ceramic layer. However, as the dielectric ceramic layers are further reduced in thickness, the electric field applied per dielectric ceramic layer is relatively increased. Therefore, an improvement in reliability in the case of applying a voltage, more specifically, an improvement in life characteristics particularly in a high temperature load test, is required for the dielectric ceramic employed.
In order to improve the reliability as described above, it has become known that it is effective to increase the grain boundary area by increasing the number of crystal grain boundaries in the dielectric ceramic. In order to increase the grain boundary area, it is necessary to reduce the grain size of the crystal grains.
A dielectric ceramic of interest to the present invention is described in, for example, Japanese Unexamined Patent Publication No. 2005-145791.
Japanese Unexamined Patent Publication No. 2005-145791 discloses a dielectric ceramic composition manufactured in accordance with a manufacturing method including a step of firing a main constituent raw material and an accessory constituent raw material. More specifically, disclosed is a dielectric ceramic composition manufactured in accordance with a manufacturing method that uses, as a main constituent raw material before firing, a barium titanate raw material powder of perovskite-type crystal structure represented by ABO3, in which the ratio A/B of an A site constituent to a B site constituent is 1.006≦A/B≦1.035 in terms of molar ratio, with a specific surface area of 8 m2/g to 50 m2/g.
According to Japanese Unexamined Patent Publication No. 2005-145791, the dielectric ceramic composition described above is intended for use as dielectric ceramic layers of laminated ceramic capacitors, which is supposed to be composed of fine grains, and to have high reliability, favorable temperature characteristics, and thus excellent high temperature load life characteristics even when the capacitor is reduced in layer thickness.
However, the example described in Japanese Unexamined Patent Publication No. 2005-145791 only discloses the thickness of the dielectric ceramic layer in the range of 2.3 μm to 2.6 μm, as shown in Tables 1 and 3 thereof. Further, Tables 1 to 3 disclose numerical values in the range of 0.141 μm to 0.284 μm as the average grain sizes for crystal grains in a sintered body of the dielectric ceramic.
Japanese Unexamined Patent Publication No. 2005-145791 thus fails to disclose any cases in which the dielectric ceramic layer has a thickness of less than 1 μm. Therefore, it is not clear that the dielectric ceramic described in Japanese Unexamined Patent Publication No. 2005-145791 still has high reliability, favorable temperature characteristics and thus excellent high temperature load life characteristics as described above, even when the dielectric ceramic layer has a thickness of less than 1 μm. Rather, it is presumed that the reliability, particularly, the high temperature load life characteristics will be likely to be degraded when the dielectric ceramic layer is reduced in thickness to less than 1 μm.